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# Voltage above 13 vdc indicator

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Hi guys

Could someone help me on a diagram how to build a module that switch a 12 vdc relay when the battery voltage is above 13 vdc.
When the battery's voltage is below 13 vdc, the relay must remain normaly open.

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How much current does the relay coil use?

The TL431 will be fine for small relays <50mA or higher with a transistor.

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Thanks for the reply. The relay needs to switch a Cole Hersee solenoid or a battery isolation relay.
I need the Cole Hersee to switch an auxiliary battery to put in charging mode. Basically the auxiliary
battery must only start to charge when the main battery voltage is above 13.5vdc.

Thanks

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I can't  help you, if you don't answer my question.

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Sorry for the confusion. The Cole Hersee solenoid will take about 5 amp to switch. But to switch the relay will take about 100mA.

Thanks

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Attached is a circuit which should do the job.

The site linked below has the formula to calculate the values of R1 and R2. R5 is just a pull-up, make it 10k and R4 limits the base current to the transistor; 2k2 will do. R3 provides hysteresis, otherwise the relay could chatter, make it 100 times the value of R1 and R2 in parallel to give 130mV of hysteresis.
http://www.reuk.co.uk/TL431-Battery-Voltage-Monitor.htm

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Thank you very much.

Will a BC557 work for the top transistor. I did the math - if I make R1 2.2K and R2 500 ohm, I will get exactly 13.5 vdc. Is it possible to replace R2 with a 1K pot (variable carbon resistor) or would it be better to put a 47E pot in serie with R2 for finetuning?
If R2 is fixed on 500 ohm, what would R3 be? Based on your instruction I understand something like this: 100 x ((2200 x 500) / (2200 + 500)) = 40740 ohm

Thank You

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The values you've chosen will work will draw a lot of current than necessary, 5mA may not sound like much but it will cost you battery life, if it's sat in standby.

I'd recommend multiplying the resistor values by 10, giving 22k, 5k and 390k (the nearest E12 value to 407.4k) and using the BC328 rather than the BC557 as with 100mA you're pusing ti close to its maximum current rating.

You can make it more accurate though.

You can download some free software to help calculate the values.

http://www.opend.co.za/software/rescalc/index.htm

For E24 values, the program gives 300R for R1 and 68R for R2. R3 would need to be 5543R but of course you'd use 30k for R1, 6.8k for R2 and 560k for R3.

If it's going to be connected to the battery permanently, you can increase the resistors further but you need to take into account that the referance (pin 1) takes 1.5uA from the potential divider so has a resistance of 1.66M which appears in parallel with R2 but it's insignificant until R2's value is over around 16k.

EDIT:
I've found a website which can do a similar thing:
http://awasteofsalt.com/voltage-divider/

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Thank very much, I will start on this project as soon as possable. You have been a great help.

Regards

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